Classifications

• • A—HUMAN NECESSITIES
  • A23—FOODS OR FOODSTUFFS; TREATMENT THEREOF, NOT COVERED BY OTHER CLASSES
  • A23B—PRESERVATION OF FOODS, FOODSTUFFS OR NON-ALCOHOLIC BEVERAGES; CHEMICAL RIPENING OF FRUIT OR VEGETABLES
  • A23B2/00—Preservation of foods or foodstuffs, in general
  • A23B2/90—Preservation of foods or foodstuffs, in general by drying or kilning; Subsequent reconstitution
  • A23B2/92—Freeze drying
• • A—HUMAN NECESSITIES
  • A23—FOODS OR FOODSTUFFS; TREATMENT THEREOF, NOT COVERED BY OTHER CLASSES
  • A23B—PRESERVATION OF FOODS, FOODSTUFFS OR NON-ALCOHOLIC BEVERAGES; CHEMICAL RIPENING OF FRUIT OR VEGETABLES
  • A23B7/00—Preservation of fruit or vegetables; Chemical ripening of fruit or vegetables
  • A23B7/02—Dehydrating; Subsequent reconstitution
  • A23B7/024—Freeze-drying, i.e. cryodesiccation or lyophilisation

Definitions

• the present invention relates to a process for producing freeze-dried strawberry slices. More particularly, this invention relates to a process for producing freeze-dried strawberry slices which are capable of rehydration in milk or other liquids within 3 minutes, while maintaining flavor, texture, and structural integrity.
• the cereal Due to the slow rate of rehydration, the cereal will be found to lose its desirable crispness and becomes mushy during the long time necessary for a reconstitution of the fruit. If the cereal is eaten before full rehydration or reconstitution of the fruit has been achieved, then the cereal will be crisp but the fruit will have an undesirable texture characterized by a dry or cottony mouthfeel. However, the rehydration rate in milk cannot be made too rapid or the fruit will lose its texture, and although the cereal will still be crisp, the fruit will be found to become mushy.
• U.S. Patent No. 3,395,022 by Vollink, et al. discloses a process for solving the problems above.
• fresh whole strawberries were slowly frozen to 0°F and held for a period of 18 hours.
• the frozen strawberries were then tempered to 25°F for 4 hours.
• the tempered stawberries were then sliced to a desired specification, then quickly refrozen in dry ice.
• the product was then freeze-dried to a desired moisture content.
• the dried strawberries rehydrated between 1 and 12 minutes in milk or other liquids. However, after holding for 10 minutes the texture of the strawberries was soft and/or slightly mushy.
• the object of the present invention is, therefore, a method for producing freeze-dried strawberries capable of rehydrating in liquids such as milk within a period of 1 to 3 minutes.
• Another object of the present invention is a freeze-dried strawberries that maintains their flavor, texture and structural integrity upon rehydration.
• a further object of the present invention is to deliver free-flowing, non-sticky strawberry slices.
• Still a further object of the invention is to produce a frozen strawberry slice in a continuous manner to be freeze-dried thereby reducing the manual labor and capital cost involved therewith.
• the present invention is concerned with a process for freeze-drying strawberries comprising the steps of: slowly and partially freezing strawberries for a time and at a temperature effective to produce a frozen water content of 25 to 45% by weight of the free water, said frozen water having ice crystals of sufficient size to substantially rupture the cellular structure of the strawberries; slicing the partially frozen strawberries; rapidly freezing the sliced strawberries for a time, and at a temperature effective to freeze the remaining free water promoting flowability and minimizing stickiness between slices; and freeze-drying said frozen, sliced strawberries to a final moisture of about 3%.
• freeze-drying signifies drying by sublimation, preferably under a high vacuum, so that the frozen ice crystals (both large and small) are removed from the strawberries by vaporization from the solid state as distinguished from the liquid or semi-solid state.
• “Slow freezing” generally involves transforming the moisture in the strawberries into a growth of large ice crystals over a period of at least 1 hour and preferably 1.5 to 2.0 hours, in order to expand and partially rupture the cellular walls or tissues of the strawberries.
• This mode of freezing is to be distinguished from normal or "rapid freezing” methods, wherein the water content of the strawberries is frozen over a period of several seconds to several minutes to form a large number of minute ice crystals which do not rupture or expand the cellular tissues of the strawberries.
• the freshly picked strawberries are subjected to freezing before they have time to deteriorate in flavor and quality.
• Whole strawberries are frozen to prevent the juice and texture loss wich occurs when cutting strawberries in the unfrozen state.
• the strawberries may then be cut after the partial freezing operation and prior to the second freezing step.
• the natural moisture content of the strawberries is in the range of about 80 to 90% and substantially all of this moisture must be completely frozen prior to freeze-drying.
• the strawberries according to the present invention are partially frozen for a time and at a temperature effective to create a frozen water fraction of 25% to 45% by weight of the free water, preferably from 30% to 40%.
• the strawberries are frozen to a temperature ranging from 16 to 32°F and preferably from 20 to 28°F.
• the time required to achieve the above frozen fraction ranges from between about 60 minutes and 120 minutes and preferably between 80 minutes and 100 minutes. This corresponds to a freezing load of about 40 to 60 btu per pound of freezable water (unbound water). Partial freezing may be accomplished by known methods of freezing.
• the water content be frozen over a sufficiently long period to develop large ice crystals.
• These large crystals will, in turn, substantially rupture the cellular structure causing large pores or channels to be formed which will increase the porosity, water removal rate during freeze-drying and the final rate of rehydration.
• the size of these large ice crystals are generally within the range of 100 to 300 microns. In cases where small ice crystals are developed the cellular structure of the tissue do not rupture or expand. In this case very few pores are created, hence, decreasing the rate of rehydration.
• Another purpose of partial freezing is to prevent juice and texture loss due to cutting the strawberries in the unfrozen state. This freezing substantially binds the free water, decreasing its availability upon cutting.
• a temperature profile in freezing the strawberries will show a gradual descending cooling curve from room temperature down to the point where moisture content of the strawberries begins to form ice crystals. At this point, the temperature curve will level into a plateau and remain at this temperature until 25% to 45% and preferably 30% to 40% of the free water crystallizes. It is essential in the present invention that the frozen water content of the partially frozen fruit does not exceed 45%, of the free water. If this occurs, too many large ice crystals are formed, resulting in a product that is very difficult to slice and shatters upon impact with a slicer. The resultant cell structure of the strawberries would be substantially destroyed resulting in the final dried product having a soft and/or slightly mushy texture upon rehydration. Comparatively, if the frozen water content is less than 25% of the free water, the time required to substantially rehydrate the freeze-dried strawberries would increase beyond the desired 2-3 minutes.
• partially freezing the strawberries to too low a temperature or at too fast a rate causes non-uniform freezing wherein the outer portion of the strawberries, namely the casing becomes hard, causing fragmentation upon slicing.
• an additional tempering step would be required for equilibration of the strawberries prior to slicing as in Vollink et al. This additional freeze thaw cycle would further deteriorate the cell structure, thus creating softer and less desirable texture upon rehydration.
• the partially frozen strawberries are sliced to a thickness ranging from 1/8 inch to 3/8 inch and preferably from 3/16 inch to 1/4 inch.
• the sliced strawberries are then rapidly frozen for a time and at a temperature effective to freeze substantially all of the remaining free water in the strawberries, i.e., to an overall frozen fraction of at least 75 to 90% by weight of the free water and preferably from 80 to 85%.
• the strawberries are frozen to a temperature ranging from 0°F to -10°F and preferably from 0°F to -5°F. It is preferred, for efficiency purposes to individually quick freeze (IQF) the strawberry slices in a fluidized air blast freezer wherein the air velocity is about 600 to 1200 ft. per minute, preferably 1000 ft.
• the air has a temperature of about -10°F to about -40°F, preferably from about -15°F to about -30°F, for a time ranging from 5 to 20 minutes, preferably from 10 to 15 minutes.
• This IQF treatment-facil itates flowability and will minimize stickiness between slices.
• This rapid freezing of the strawberry slices develops small ice crystals as the remaining free water is frozen. These small crystals will cause small or no pores to be formed, having little or no effect on the final structure upon rehydration. These small ice crystals also have little or no effect on the cell structure.
• the degree of cell destruction is dependent on only the initial slow freezing step of the whole strawberry, whereby large ice crystals are created which rupture the cell structure. The degree of rupture is a direct relationship to the amount of water frozen in the slow freezing step. The more rapid second freezing step has no real effect on the structure because only small ice crystals are formed. Therefore, it is believed that the final product has about 40% of its cells destroyed due to the formation of large ice crystals while the remaining 60% remains substantially intact.
• the frozen strawberries are then freeze-dried by any known commercial means, wherein the ice crystals are sublimed under vacuum until a terminal moisture content of 1% to 5%, and preferably about 3% is obtained.
• the sublimed large ice crystals due to their size, leave large channeled openings in the strawberries as the crystals vaporize, thereby enable quicker penetration when exposed to a rehydrating medium.
• the dry strawberry slices can be consumed as is or may be combined with a dry breakfast cereal food such as corn flakes or other rehydratable products adapted to be consumed with milk or cream.
• a dry breakfast cereal food such as corn flakes or other rehydratable products adapted to be consumed with milk or cream.
• the strawberry slices of this invention due to their extreme porosity are extremely hygroscopic.
• the dried strawberry slices therefore, should be protected from atmospheric and in-package moisture and at no time should the moisture content be allowed to exceed about 4%.
• a charge of freshly picked strawberries were washed and then graded for uniformity.
• Strawberries having a size of about 5/8 to 1 1/2 inch in diameter were placed on a conveyor traveling through an air blast freezer exposing the strawberries for about 90 minutes to a stream of air having a temperature of about 20°F and traveling at a velocity of about 250 ft. per minute.
• the average temperature of the partial frozen strawberries exiting the freezer was about 28°F.
• a frozen water fraction of about 40% by weight of the free water was achieved.
• the partially frozen whole strawberries were cut into 1/4 inch thick slices.
• the partially frozen strawberry slices were then fed to an IQF freezer i.e, fluidized blast air, to completely freeze the remaining free water in order to prevent stickiness between the slices during storage and to prepare for freeze drying.
• the average freezer temperature was about -20°F with air velocity of about 900 ft. per minute and the freezing time was about 12 minutes.
• the product had a final frozen water content of about 80% by weight of the freezeable water (both large and small) ice crystals.
• the strawberries were then freeze-dried by ' conventional means to a moisture content of about 3%.
• the freeze-dried strawberry slices were then combined with cornflakes at a level of about 10% by weight strawberries and about 90% cornflakes.
• the blending operation was conducted in a packaging room having a relative humidity of 30% in a period of less than 10 to 15 minutes thereby limiting moisture pickup of the strawberries to less than 1%.
• the strawberry and cornflake mix was then packaged in a water resistant, wax laminated foil liner which was placed inside a chip board shell and then closed with a wax laminated over-wrap.
• the packaged product having a terminal moisture content of less than 2.5% for the strawberries and about 2% for the cornflakes was stored in this form at 70°F and 50% relative humidity for 3 to 6 months without any degradation of product quality.
• the strawberries were found to reconstitute in milk or cream in about 90 seconds to a flavor, texture and appearance close to that of fresh berries.
• the breakfast cereal could be eaten within lk to 3 minutes with the berries being fully reconstituted and maintaining their structural integrity while the cereal remained in its crisp form.

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• Life Sciences & Earth Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical & Material Sciences (AREA)
• Wood Science & Technology (AREA)
• Zoology (AREA)
• Food Science & Technology (AREA)
• Polymers & Plastics (AREA)
• Chemical Kinetics & Catalysis (AREA)
• General Chemical & Material Sciences (AREA)
• Storage Of Fruits Or Vegetables (AREA)

Applications Claiming Priority (2)

Publications (1)

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Family Applications (1)

Country Status (2)

Cited By (9)

Families Citing this family (1)

• 1985
  • 1985-06-25 EP EP85304509A patent/EP0166600A2/fr not_active Withdrawn
  • 1985-06-25 DK DK286885A patent/DK286885A/da not_active Application Discontinuation

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